A piezoelectric sensor is a device that uses the piezoelectric effect to measure pressure, acceleration, strain or force by converting them to an electrical charge.
A piezoelectric sensor that can be used to measure pressure, force or strain under a surface (overlay) is known from published patent application WO 2011/158 154 A2 (hereinafter the '154 patent), for example. The piezoelectric sensor comprises a layer of piezoelectric material sandwiched between a PCB and an electrically conductive foil. The electrically conductive foil is attached to the overlay by means of an adhesive layer.
Force, pressure applied to the overlay, or the strain of the overlay, tends to cause a change of shape in the layer of piezoelectric material (generally, the force or pressure tends to bend the layer). The change of shape causes an electrical charge, which is collected via an electrical circuit to signal analysing unit after being converted to digital format. The signal can be analysed by the signal analysing unit, and so the piezoelectric sensor can be used as a piezoelectric touch switch. The piezoelectric touch switch of the kind described in the '154 patent is widely used in different kinds of apparatuses and installations.
It is known that the overlays used in apparatuses and installations can be selected from a very large group of materials and qualities, each having different physical properties. The thickness of the overlay and the material, of which the overlay has been made, for example, can be different for each kind of apparatus or installation. Furthermore, the elasticity of the overlay (i.e. how much the overlay bends if a given force or pressure is exerted on it) may be different. The thickness or elasticity may depend on the position where the piezoelectric sensor switch is assembled, for example, structures show a smaller elasticity close to corners. A displacement of a few millimeters can influence the elasticity or stiffness significantly, in particular if the displacement is towards a stabilizing structure such as a ridge or corner (or further therefrom).
For these reasons, a lot of matching of the piezoelectric sensor set-up between different kinds of apparatuses and installations is currently required in order to have the piezoelectric sensor to work properly. Furthermore, a certain amount of matching may be necessary even between different units of the same make of the apparatus or installation. In particular, if the piezoelectric sensor is a piezoelectric touch switch, tuning is normally required in order to have the switching threshold defined. The aim of the matching is to have the switch not to switch too early (to avoid registering noise as switching, i.e. avoiding false alarms) and to have the switch to switch when a high enough pressure, force or strain is exerted on the overlay (to enable registering a reasonable pressure, force or strain as a valid switching command).